The invention relates to a steel cord for the reinforcement of elastomers, comprising two strands of at least two filaments each so as to form an m+n -structure, where m is the number of filaments of the first strand and n the number of filaments of the second strand, m and n being greater than or equal to two.
The steel cord according to the invention is particularly suitable for use as a reinforcement of rubber articles such as tires, and more particularly for use as a reinforcement of breaker layers in a tire.
Steel cords for use as a reinforcement of breaker layers in a tire conveniently comprise steel filaments having a diameter between 0.05 mm and 0.60mm, preferably between 0.15 and 0.45 mm. A conventional steel composition for such steel cords is a carbon content above 0.65 %, preferably above 0.80 %, e.g. 0.83 % or 0.85 %, a manganese content between 0.40 and 0.70 %, a silicon content between 0.15 and 0.30 %, and maximum sulphur and phosphorus contents of 0.03 %. However, the invention is not limited to such a steel composition. Other elements such as chromium, nickel or boron may also be added. The steel cord usually has a rubber adherable layer such as a copper, zinc, or brass alloy.
The state of the art of steel cords for reinforcement of elastomers, and more particularly for reinforcement of a breaker layer of a tire provides several different constructions.
Among these constructions the n.times.1 -constructions occupy a special place. These are constructions with n filaments twisted together with the same twist pitch and in the same twist sense, n is an integer number between 3 and 5. The problem with these constructions is that they have a central void where rubber cannot penetrate during vulcanisation and where moisture may easily enter and cause corrosion.
A solution to this problem has been given by the open n.times.1 -constructions. These are constructions where one or more filaments are kept apart from each other by giving them a specified preformation during the twisting process. However, this preformation must exceed a certain limit in order to avoid closing the steel cord when this is put under tension during the vulcanisation process. The problem is then that too high a preformation may cause an irregular cord aspect and instability.
In addition to the n.times.1 -constructions the 2+2 -construction which is disclosed in US-A-4,408,444 has been widely used in the tire manufacturing industry too. This cord has the advantage of having full rubber penetration whether brought under tension or not, but has the drawbacks of a poor fatigue limit and a still too great cord diameter. As a consequence this cord is less suitable when a high fatigue performance is required or when a thin rubber ply is a priority.